Simulation of Oxygen Isotopes in a Global Ocean Model
A. Paul*, S. Mulitza, J. Patzold and T. Wolff
Universitat Bremen, Fachbereich Geowissenschaften, Postfach 330440,
D-28334 Bremen, Germany
*corresponding author (e-mail}:apaul@palmoduni-bremen.de
Abstract: We incorporate the oxygen isotope composition of seawater OI'Ow into a global ocean
model that is based on the Modular Ocean Model (MOM, version 2) of the Geophysical Fluid
Dynamics Laboratory (GFDL). In a fIrst experiment, this model is run to equilibrium to simulate the
present-day ocean; in a second experiment, the oxygen isotope composition of Antarctic Surface
Water (AASW) is setto a constant value to indirectly account forthe effect of sea-ice. We check the
depth distribution of OI'Ow against observations. Furthermore, we computed the equilibrium
fractionation of the oxygen isotope composition of calcite 0 1 '0, from a paleotemperature equation
and compared it with benthic foraminiferal 0 1 '0. The simulated OI'Ow distribution compares fairly
well with the GEOSECS data. We show that the OI'Ow values can be used to characterize different
water masses. However, a warm bias of the global ocean model yields 0 1 '0, values that are too light
by about 0.5 %0 above 2 km depth and exhibit a false vertical gradient below 2 km depth. Our ultimate
goal is to interpret the wealth of foraminiferal 0 1 '0 data in terms of water mass changes in the
paleocean, e.g. at the Last Glacial Maximum (LGM). This requires the warm bias of the global ocean
model to be corrected. Furthermore the model must probably be coupled to simple atmosphere and
sea-ice models such that neither sea-surface salinity (SSS) nor surface Ol'Ow need to be prescribed
and the use of present-day ol'Ow-salinity relationships can be avoided.
Introduction
Isotopes in the Hydrological Cycle
Measurements of the isotopic composition of water at the various stages of the hydrological cycle
has enabled the identification of different water
masses and the investigation oftheir interrelationships; measurements of the variations of the isotopic composition of proxy materials in climate archives such as ice and sediment cores has become
a most useful tool in paleoclimate reconstructions
(Gat 1996; Schotterer et al. 1996).
The isotopic species ofthe water molecule that
are of most interest to the geosciences are
IH2H I6 0, IH 2
18 0 and IH3H I6 0. A water molecule
containing the radioactive isotope 3 H (tritium) or
either ofthe stable isotopes 2H (deuterium) and 18 0
(oxygen-IS) is heavier than the water molecule
IH 2 W 6 0 that is by far the most abundant in the
natural assembly of all isotopic species. Therefore,
water vapor evaporating from the sea surface is
depleted of heavy isotopes relative to ocean water,
while rain precipitating from a cloud is enriched
relative to the cloud moisture. Hence whenever a
water sample undergoes a phase transition (e.g.
evaporation or condensation), temperature-dependent isotope fractionation occurs (Fig. ). In the case
of tritium, this effect is generally masked by mixing
water from different sources (JouzeI19S6). But in
the case of deuterium and oxygen-IS, the isotopic
composition of precipitation shows a large variation
with latitude, height and continentality. The depletion of precipitation in deuterium and oxygen-IS is
most pronounced in the polar regions. The variation
in the isotopic composition of seawater is comparatively small and mainly determined by freshwater
input and mixing between water masses.
From FISCHER G, WEFER G (eds), \999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 655-686
A. Paul*, S. Mulitza, J. Patzold and T. Wolff
Universitat Bremen, Fachbereich Geowissenschaften, Postfach 330440,
D-28334 Bremen, Germany
*corresponding author (e-mail}:apaul@palmoduni-bremen.de
Abstract: We incorporate the oxygen isotope composition of seawater OI'Ow into a global ocean
model that is based on the Modular Ocean Model (MOM, version 2) of the Geophysical Fluid
Dynamics Laboratory (GFDL). In a fIrst experiment, this model is run to equilibrium to simulate the
present-day ocean; in a second experiment, the oxygen isotope composition of Antarctic Surface
Water (AASW) is setto a constant value to indirectly account forthe effect of sea-ice. We check the
depth distribution of OI'Ow against observations. Furthermore, we computed the equilibrium
fractionation of the oxygen isotope composition of calcite 0 1 '0, from a paleotemperature equation
and compared it with benthic foraminiferal 0 1 '0. The simulated OI'Ow distribution compares fairly
well with the GEOSECS data. We show that the OI'Ow values can be used to characterize different
water masses. However, a warm bias of the global ocean model yields 0 1 '0, values that are too light
by about 0.5 %0 above 2 km depth and exhibit a false vertical gradient below 2 km depth. Our ultimate
goal is to interpret the wealth of foraminiferal 0 1 '0 data in terms of water mass changes in the
paleocean, e.g. at the Last Glacial Maximum (LGM). This requires the warm bias of the global ocean
model to be corrected. Furthermore the model must probably be coupled to simple atmosphere and
sea-ice models such that neither sea-surface salinity (SSS) nor surface Ol'Ow need to be prescribed
and the use of present-day ol'Ow-salinity relationships can be avoided.
Introduction
Isotopes in the Hydrological Cycle
Measurements of the isotopic composition of water at the various stages of the hydrological cycle
has enabled the identification of different water
masses and the investigation oftheir interrelationships; measurements of the variations of the isotopic composition of proxy materials in climate archives such as ice and sediment cores has become
a most useful tool in paleoclimate reconstructions
(Gat 1996; Schotterer et al. 1996).
The isotopic species ofthe water molecule that
are of most interest to the geosciences are
IH2H I6 0, IH 2
18 0 and IH3H I6 0. A water molecule
containing the radioactive isotope 3 H (tritium) or
either ofthe stable isotopes 2H (deuterium) and 18 0
(oxygen-IS) is heavier than the water molecule
IH 2 W 6 0 that is by far the most abundant in the
natural assembly of all isotopic species. Therefore,
water vapor evaporating from the sea surface is
depleted of heavy isotopes relative to ocean water,
while rain precipitating from a cloud is enriched
relative to the cloud moisture. Hence whenever a
water sample undergoes a phase transition (e.g.
evaporation or condensation), temperature-dependent isotope fractionation occurs (Fig. ). In the case
of tritium, this effect is generally masked by mixing
water from different sources (JouzeI19S6). But in
the case of deuterium and oxygen-IS, the isotopic
composition of precipitation shows a large variation
with latitude, height and continentality. The depletion of precipitation in deuterium and oxygen-IS is
most pronounced in the polar regions. The variation
in the isotopic composition of seawater is comparatively small and mainly determined by freshwater
input and mixing between water masses.
From FISCHER G, WEFER G (eds), \999, Use of Proxies in Paleoceanography: Examplesfrom the South Atlantic. Springer-Verlag
Berlin Heidelberg, pp 655-686